Automobile exhaust gas purifers such as manifold reactors or catalytic converters should comprise a heat insulator which can withstand high temperature, because the inside of such a device has to be kept warm so that its exhaust gas purifying ability may be improved and the heat released from such a device has to be prevented from affecting adjacent parts of the automobile.
The so-called ceramic fibers, which are fibrous heat insulators for high temperature use constitute one of the materials available for this purpose. Fibers of alumina-silica can withstand a maximum working temperature of 1200.degree.-1400.degree. C.; one of silica can withstand 1000.degree. C.; one of potassium titanate can withstand about 1000.degree. C.; and one of zirconia can withstand 1800.degree. C. Slag wool is also available, but the working temperature it can withstand is low, i.e., about 600.degree. C.
These fibrous materials have a heat insulating ability two to three times as high as that of heat resistant, heat insulating brick; a bulk specific gravity of 0.05-0.25 g/cm.sup.3, which is about 1/8 of that of the heat insulating brick; and are flexible and vibration-resistant, so as to be quite free from the possibility of being broken by heat shock. Being less resistant to wind velocity, however, they usually need a heat-resistant metal plate applied on the heating surface and are sandwiched between the metal plate and the outer shell, when they are used in a manifold reactor. When they are used in a catalytic converter, they fill the space between the catalyst carrier and the outer shell. In any case, the fibrous heat insulator has to fill a very narrow space. To do this efficiently without sacrificing performance, various methods have been worked out. To give some examples, there are:
1. The method of inserting a bulky heat insulator through the end of the space between the heat insulating inner cylinder and outer cylinder;
2. The method of introducing into the outer cylinder an inner cylinder wrapped with a felt-like heat insulator, or sheathing the inner cylinder with an outer cylinder split into two parts; and
3. The method of inserting a stainless steel, foil-packed fibrous heat insulator. There are, however, many drawbacks in these methods. For example:
1. The efficiency is poor;
2. The fill density becomes uneven; and
3. It is expensive.